


Catching a Thief

by TwistedPancakes420



Category: Undertale (Video Game)
Genre: Dubious Consent, F/M, Female Reader, Fingering, Only A Little Plot, Rough Sex, Shameless Smut, Underfell, thief reader
Language: English
Status: In-Progress
Published: 2019-03-26
Updated: 2019-04-04
Packaged: 2019-12-18 11:27:22
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 3,871
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18248906
Author URL: https://archiveofourown.org/users/TwistedPancakes420/pseuds/TwistedPancakes420
Summary: The Underground is a hard place to survive. Especially when you're a human. Resources are limited, unless you know how to pick locks. Desperation did shit to people.





	1. Chapter 1

Desperation did shit to people. It made them make really really stupid choices. Here you were, standing outside of a somewhat notorious monster house. The ‘Great’ and Terrible Papyrus lived there. The house wasn’t too fancy. Nice, but no real security. Breaking in shouldn’t be too hard. It would serve him right. That skeleton was the guard of this town. Always looking for humans to send to Asgore. You just so happened to be one of those humans. Surviving in this underground hellhole was hard enough, avoiding monsters as often as possible and still managing to find a way to eat was a full time job. Papyrus’s house would be sure to have some supplies to help with the journey to the barrier, why not just take it?

You scoped out the place for a couple of hours, making sure that he had already left for his evening rounds. Quickly you jumped the fence to the backyard. The back door was locked but it only took a few minutes to pick the lock. You snuck inside to what looked like a kitchen. The place was dark but you could pick out a few shiny medals of his that could easily be sold on the table. Quickly, you open your backpack and tossed them inside. Looking around you found the stairs and tiptoed up. The first room at the top of the stairs was obviously his, as he conveniently labeled it.

You open the door. Jackpot. 

There were a few knives on his nightstand, a warm looking jacket hung in the closet, and another backpack filled with other survival supplies. Gladly you shoved it all in your backpack. The last place you wanted to hit was the kitchen for whatever food was in there and you could get the hell out of here. 

You opened the door and quietly shut it behind you, as it clicked quietly shut gravity suddenly became heavy. Like unbearably heavy. You fall to the floor with a thud and an _oof._

“Well well well… isn’t this adorable.” a new voice, one you never heard before spoke, it wasn’t quite as rough as Papyrus’s but it held a level of amusement.

You manage to look up to see another skeleton. He wasn’t as tall as Papyrus but that wicked grin on his face sent a chill down your spine. Did he have a brother? How did you not know this before hand?  
“Fuck…” you groan and realize the position your in. “Please… don’t tell Papyrus… Please, he’ll kill me.”

He laughed. “You don't wanna die? Kid, I gotta live with him. If he found out I helped you get away what do you think would happen? I'm gonna need more than a little _please_ to convince me.” he walked in front of you until his shoes were the only thing in view. The magic he was using let you go only to be replaced with his boney hand grabbing you by the hair and yanking you up. He was ridiculously strong.

You cried out but it seemed he didn't care at all as he dragged you down the hall and in through a different door. He pushed you in roughly causing you to crash onto the floor. You did your best to crawl away but you quickly realized there was no other exit than the door you came through. That door was slammed shut and there was a click of the lock. You looked back at him, now seeing more features. Sharp teeth with one being gold, red eyes, that terrifying grin and hungry look made him distinct from his brother but you felt the same fear.

Again he grabbed at you, this time forcing your backpack off your shoulders. He opened it and spilled the contents onto the floor to inspect it all. He chuckled. “That's it?” He picked up the hunting knife and inspected it. “You aren't very good at this whole robbing thing are ya?”

“I just want to leave here. I'm trying to go home.” Your voice was small, timid.

He rolled his eyes. “Don’t you get it? The underground is your ‘home’ now. There isn't a way out. Get fuckin used to it.”

Slowly you started standing up. “Look I don't know what you want… just please don't turn me in. I’ll leave the stuff here.” You remembered your last encounter with Papyrus, he managed to beat you within an inch of your life before you somehow got away. That was something you didn't want to do again. God only knew what this guy was capable of, you didn't want to find out.

He pointed the knife at you casually and his eyes lit up with a deep crimson. “Back on your knees.”

You didn't hesitate. “What do you want? I'll leave tonight, make sure you never see my face again. Just please…” you begged, pathetically. 

“Maybe.” You saw a red tongue come out and lick the outside of his teeth. “But I gotta do something about this. No crime can go unpunished after all." 

That look in his eye would be pretty sexy had it not been coming from this skeleton. Against your better judgement you still ask, “What… what do you want?”

“I want you to be a good girl and start taking off those clothes.” He leaned back on the door frame and watched you expectantly.

You blushed before getting up and backing yourself up to the other end of the messy room. “What? No way! You can't be serious!” You protest. 

He frowns. “I ain't going to ask again, dollface.” He twirled the knife in his fingers. “Drop the clothes or I'll just call my brother home now. I'm sure he'll be real pleased about what I found skulking around.” He smirked. 

There was another few moments hesitation. He was being deadly serious, and you didn’t want to find out how well he could use that knife. You took off your coat, followed by your socks and shoes then your sweater and jeans. That left you in your bra and panties. You looked up at the skeleton and he gestured for you to continue. He seemed transfixed on your figure. He was obviously holding himself back from taking you right then and there. No, he wanted to have a little more fun before that. 

With an embarrassed blush you unclasped the back of your bra and it fell to the ground. You did your best to cover yourself up as you fiddled with your panties. 

“Don't cover yourself.” He commanded. “I want to see everything.” 

Slowly you took your arm away from your chest, revealing yourself. There was another shiver but not one you expected. You were afraid but at the same time some part of you was excited. You weren't sure how to feel about it. 

“Also dollface…” he stopped you from pulling down your panties. “Turn around and take them off slowly.” He ordered. 

Not wanting to aggravate the man with the knife you did what he said. Turning and slowly leaning down as you slid them down your legs. You completely exposed yourself to him. 

“Oh _fuck_ yeah.” You didn't hear him cross the room but suddenly he was behind you and giving your ass a firm slap. “I don't know what it is about humans but hot damn.” 

You yelped as the hand came down hard causing you to stand up straight. He immediately pushed your upper half down again and continued groping your ass. You braced on the wall in front of you for support and he manhandled you.

You held back a semi pleasurable whimper. That became even more hard to do as you felt the tip of his finger go down and start teasing your pussy roughly. You bit your lip hard. 

There was another dark chuckle. “You feel pretty wet, kitten. You like it when I play with you?” He slaps your ass again but harder.

This time you couldn't stop the whimper. “Fuck~.”

That finger slowly slipped inside you. A light moan came out of your lips. 

“Yeah that's what I thought, slut.” He inserted another finger and began his assault on your wet core.

You hated how he was hitting all the right buttons on your body. He even reached around and groped at your breast. It felt so fucking good, you couldn't hold back the small moans from coming out. Quickly you felt that pressure start building in the pit of your stomach and began to lean into him. 

“I couldn't have asked for a better human slut.” He pinched at your nipple. “By the way, my name is Sans. I'm about to make you start singing it.” He sounded so confident in that statement but damn he had the skills to back it up. He managed to find your G spot and abused the hell out of it, sending you over the edge. 

“Fuck yes Saaans~...” You moan loudly and scratch at the wall. Your legs started shaking, the only reason you didn’t collapse was because he was holding you up. 

He took his fingers out. There was part of you that felt shame for just letting him do this to you but on the other hand you've been pent up for weeks. Maybe it wasn't so bad taking a little pleasure from this place. Even if it was at the hands of this skeleton.

He grabbed at your hair again and dragged you to his bed. You landed on your back and looked up in time to see him taking off his coat and shirt before dropping his shorts revealing a decently sized red cock. 

You blushed again and knew that it would be a tight fit but fuck it. You welcomed it, no, right now you _needed_ it.

He still had that same smirk on his face as he approached and climbed on top of you. Effectively getting between your legs. He lined himself up and roughly slammed into you, causing a growl to come from him. “Fuck you’re tight…” 

You cried out, the pain and pleasure mixed together and you couldn't stop yourself from clawing at him. “Too fast!” You choked out.

He took both of your wrists and easily pinned them above your head. He didn't seem to care about your protests at all. All he did was go harder.

You squirm around for a little bit but slowly the pain grew to pleasure and you relaxed against him. Once again moaning for him.

He let go of your wrists and used his hands to grip your hips. You grip the sheets, once again you felt a climax coming. 

A hand slipped up around your neck and squeezed just a little. That got your attention. 

“I love that look in your eyes. You want it so fucking bad. You dirty fucking whore.” He leaned down and started licking and biting at your neck. 

You moaned louder. “D-Don't stop! Oh fuck please Sans don't stop!” You beg and grip at his ribs. Your mind was completely clouded by ecstasy. No rational human would be begging for anything but their life from this monster. Yet here you were moaning like you were in heat. 

“I don't plan on it, doll.” He was practically drilling you into the matress. Watching you wriggle under him and cry out in pleasure drove him crazy. 

You couldn't hold yourself back anymore. Again you cum for him and start screaming his name. “SaaaAAAANNSSS~!!” 

Something about that sent him over the edge too and you feel him unload inside of you with a growl. You both ride out your orgasms before he gets off and lays down next to you. 

Neither of you move as you both lay there panting. Eventually you look at each other. “I'll let you of with a warning this time, dollface. Get your shit and get out before Paps comes home.” too bad… fun times are over again.

With shaking legs, you get up and redress yourself before shoving what was in your backpack in there again. He watches you with a smirk on his face.

“Hey dollface.” He calls to you and you look back. “If I see you out there… you might have to convince me to keep it to myself again.” He winks. 

There's heat on your face. “I'll… Keep that in mind.” finally you leave and manage to sneak out just before the bigger skeleton comes home. Overall… that went way better than expected.


	2. Chapter 2

**Notes for the Chapter:**

> The unexpected return! Yeah I kinda had a couple more ideas I wanted to fuck around with in this story. Enjoy, ya filthy sinners ;P

It had been a week at least since breaking into the skeleton brother’s house. It had been a great idea, you were doing pretty good considering your circumstances. You had even made it to Waterfall. In that time you had learned a very valuable piece of information. How to leave. The only problem was how to execute the plan. 

For now, that could wait until you found some more food. The crap pasta you took from Papyrus was pretty much gone now. Of course you could trek back to Snowdin and take some more from that damn skeleton’s fridge.

Or you could see if there was any food stashed in this sentry station. No one was there and it was locked up for the night. Why not?Carefully you approached the side door of the closed up hut. You started picking the lock after a quick look around for any prying eyes.

It wasn’t hard to bend the lock to your will. It was satisfying to hear it click open. You stand and just as your hand touches the door a hand appears on your shoulder. A deep chuckle follows it. “Look what I found. A lost kitten.” 

Slowly you turn to see Sans staring at you with a devilish grin. You swipe his hand off of you and turn. “What the hell are you doing here?” You press your back against the door.

“Surprisingly… my job.” he points at the hut. “Looks like you got busted. Again.” he laughs. “I knew you sucked at this but this has to be a joke. Or…” his finger trails from your collarbone to your chin, which he grabs and pulls your face closer to him. “You just wanted round two?”

Your face heat up. “Why would I want that?” 

He smirked. “Because you liked it.” he leaned in closer. “Because I can bet you’ve been thinking about it since you left.” his other hand crept up the side of your shirt and bra and cupped your breast, giving it a hard squeeze.

You gasped out of reflex. 

The door swung open and you were pushed inside. You stumbled back until you hit the opposite wall. Without missing a beat, Sans followed you in and slammed the door shut with magic. Again you stand here, completely at his mercy. Not that you minded too much. He was right when he accused you of thinking about it. Not that you would let him know that.

With a huff you cross your arms. “So are you going to turn me in this time?”

“All depends on you, sweetheart.” he shrugs off his jacket. “Do you think this crime needs some form of punishment?” he flashed that sexy smirk.

You felt weak at the knees and you bit your lip. “What do you want?” your jacket slipped to the ground along with your backpack. You could feel your face burning but your desire egged you on. 

“That’s a good girl.” he chuckled. “On your knees.”

You sunk down to your knees and looked up at him for your next command. There was a certain excitement to being around this skeleton that you didn’t quite understand. It felt so good though, you could feel yourself getting wet. 

Sans stood right in front of you, looking down. Those red eyes watched you with great interest and his hand brushed your cheek tenderly. “Get that top off too, just in case this gets messy.”

You rip your top off and see that he seems very pleased that you didn’t bother with wearing a bra today.

“Naughty girl…” he pulls down his shorts, his dick already formed and hard. “Open wide.”

You hesitate only for a moment before opening your mouth for him. He grabs the back of your head and shoves his cock into your mouth. You nearly choke since it took you by surprise but you quickly recover and are able to match pace with his thrusts into your mouth.

He moans quietly. “Oh fuck yeah, babe…” 

You slip your hands down into your pants, the heat between your legs was starting to become damn near unbearable. 

The hand on the back of your head grabs a bunch of hair and forces you to look up at him. His dick comes out of your mouth, making a wet sound.

“Did I say you could touch yourself, slut?” he growls.

You stare up at him in confusion. “N-no…”

“Only I get to decide if you feel good.” He let go. “Back to work.”

You whined as you took your hand out of your pants and went back to sucking his cock. You couldn’t stand that he was getting you so worked up but could do nothing about it. You squirmed on your knees. It just makes him buck harder into your mouth.

Sans chuckles again. “What’s got you so hot and bothered? You that desperate to get fucked?” he pulls away.

You take a breath and wipe your mouth.

“Well?” he insisted.

You blushed again. “I-I… um… y-yes.” you mutter.

He puts a hand to the side of his face. “I’m sorry I couldn’t hear my pet beg. Care to speak up?” he mocked.

You grit your teeth, you could just slap him. No doubt that would earn you some new form of punishment and you felt so horny, you fucking needed some form of release. So you gave in to him with a whine. “Sans I need you so fucking bad. Please please fuck my pussy.”

He shivered, the sound of his bones rattling followed. “I could listen to that shit all day.” with a grin he yanks you up you and bends you over the counter, right in front of the closed shutters that led outside. No one would be able to see in unless they really tried.

You whimpered as you felt the tip of his cock at your entrance.

“Try not to be to loud, pet. It’d be a real shame if we got caught.” he shoved himself deep into your core.

You gasped and put your hand over your mouth to keep your moans from coming out.

It was easier to adjust to him this time so when he started drilling into you it felt like exactly what you needed. It became increasingly harder to hold yourself back.

His hand came down on your backside and you squeaked.

“Hush, pet.” he teased and struck your ass again. All you could do was hold back your moans but you could feel your climax building.

His hand crept up your back and found its way to the back of your head again, tangled in your hair and he pulled again, the other hand kept hold of your hip.

At this point you couldn’t even hold back whimpering. He was hitting your g-spot so much better in this position. His thrusts became faster. “Fucking cum for me, pet.” he commanded breathlessly.

That pushed you over the edge and you felt your pussy clamp down on him. There was no holding back from moaning. Damn anyone who heard you. “Fuck Sans~! Yeeeesss!”

He groaned hungrily and hilted completely in you, filling your pussy with his own cum.

You both rode out your orgasms before he pulled out and put his shorts back on.

He laughed. “You couldn’t help yourself could ya, sweetheart?” he helped you pick up your clothes. “Don’t worry too much, not a lot of people travel this way anyway.”

You sigh in relief. “So let me guess, you want me to get out now?” you say a little sadly as you start putting your clothes on.

He watches. “You don’t have to. If you stick around I can bring some Grillby’s. Bettin’ it’s been awhile since you’ve had a hot meal. Got somethin’ else for you too.”

You just finished buttoning your pants. “For me?”

“Yeah… give me a few minutes. I’ll be back.” and just like that he disappeared.

You finished getting dressed and looked around the small hut. There was nothing there, it was a small space, sure, but you thought that maybe there would be something here. Ah well… at least Sans was bringing a hot meal. Its been way too long since you had some proper food.

When he reappeared he held a paper bag and a couple of drinks. “Miss me?”

You divided the food between the two of you, he brought back a couple of burgers. Definitely not complaining about that.

Sans sits on the floor at the opposite side of the hut, giving you both a little more room to eat.

“So… you said you had something for me?” you ask about halfway through the meal, breaking the silence.

He nods. “Yeah, I do.” he picks up a bottle of mustard and drinks from it. You decide not to mention it. “I’ve been thinking about it since last time, and I think it’ll be beneficial for both of us.” he grins.

You shove a few fries in your mouth. “How so?”

Sans reaches into his pocket and pulls out a collar. “You put this on and keep it on.”

You clumsily catch it and look it over. Its a pretty simple design, a leather collar with a silver ring on the front of it. You narrow your eyes at the skeleton. “Why would I do that?”

“It’ll help me keep track of you. You get in trouble? I can come help, from the bruises and cuts I saw on that soft skin of yours, I'm bettin' you'll need it. You feeling frisky again? Hell yeah, I’ll come to help.”

You set the collar aside and continue eating. “What’s in it for you? I’ve been told you have a reputation for being a lazy ass. Why go through the effort?”

His sharp teeth rip through part of the burger. “Ya see, I was there when Alphys made the discovery that it takes a monster and a human soul to get through the barrier. I’m betting you’ve already heard about it?”

You had indeed. Thinking through it, this solved your problems. “You don’t want to leave with your fellow monsters? Or your brother?”

He shrugged, “I’m sure they’ll make it someday. Some other stupid human will fall down here and Paps can tear them apart. When you're down here long enough, ya just become... opportunistic. Fuck it, am I right?”

You finished your burger and eyed the collar again.

“Of course, you can say no to this offer and we’ll just go back to our little game of cat and mouse.” he smirked.

Your eyes flick to Sans. “Alright.” you pick it up and put it on. “Fuck it, am I right?”

He chuckles. “That’s the spirit, pet.”

**Author's Note:**

> So I kind of wrote this on a whim, l hope you guys liked it. I'm thinking about continuing the story, mostly it would just be more smut but ya know. Let me know what you all think.


End file.
